This invention relates to enhanced definition television (EDTV) systems.
Recent interest in producing high quality television images has generated many proposals for improved television systems. One of the better known examples of such a system is a high definition television (HDTV) developed by the Japanese. In comparison to the existing NTSC receivers, the Japanese HDTV offers better spatial resolution, a wider aspect ratio and it is free of other defects associated with NTSC systems.
For many in the broadcast industry, however, the Japanese HDTV has a major drawback. It is incompatible with the existing 160 million NTSC receivers. A commonly held view is that the early changes in television broadcast systems should be receiver-compatible, i.e., they should be compatible with the current NTSC receivers, so that broadcasters who adopt the new systems will be able to continue to serve the large number of NTSC receivers. Of course, the standard receiver would display a standard image and not an enhanced image. Responsive to this concern, some people have proposed that the industry start with an intermediate quality, enhanced definition (EDTV) system which is receiver-compatible and then move to a non-compatible system at a later stage. That is, the EDTV system would be a technological "bridge" from the current system to a final system. Ideally, the EDTV system would be compatible with both the current NTSC system and the final system so that consumers would not find themselves owning receivers that were incapable of receiving programming transmitted according to the final system's standards.
Generally, the broadcast industry agrees that the final system will have a wider aspect ratio than the NTSC system, probably more like the aspect ratio that is available in widescreen movies. Thus, instead of the 4.times.3 frame of the NTSC system, the final system would produce a 16.times.9 frame. The top and bottom regions of the NTSC frame, representing about 25% of the image area, will be unused in the final system.
One approach to achieving backward compatibility in the EDTV systems is to transmit the 16.times.9 frame using an NTSC signal and to usurp the "unused" portions of the 4.times.3 NTSC frame for transmitting image enhancement information for the EDTV receiver. The NTSC receiver displays only the NTSC signal and ignores the information in the enhancement signal. Thus, the image on the NTSC receiver is normal throughout the NTSC portion and is blank in the "unused" top and bottom portions. Although such a system would have the receiver-compatibility desired, the presence of the blank areas at the top and bottom of the screen is thought by some to be undesirable.